1. Field of Invention
This invention relates to pulsed electric gas discharges and more particularly to preionization and switching of such discharges.
2. Description of Prior Art
The various arts dealing with pulsed gas discharges have long been known. However the recent advent of high powered pulsed gas lasers, and particularly the transversely excited, atmospheric pressure (T.E.A.) carbon dioxide laser has led to the development of high pressure large volume pulsed glow discharges. In gas lasers the function of the electric discharge plasma is to excite atoms or molecules of a lasing gas to higher energy states. This function may be achieved by the pulsed glow discharge which produces a large uniform volume of plasma uniformly attached to the electrodes of the discharge and which has a high electron temperature. However at atmospheric pressure or thereabout the pulsed glow discharge is inclined to arcing unless otherwise constrained. On arcing the discharge current becomes highly localized into a filamentary conducting channel which is unsuitable for laser excitation. Several means of suppressing the arcing of the pulsed glow discharge have recently been developed. In the glow discharge the ionization of the gases is provided directly by the current passing through the gases. At high pressures a tendency exists for the direct ionization to occur nonuniformly throughout the discharge volume which eventually leads to the highly ionized filamentary volume which is the arc. If, however, auxilliary means of ionization is provided which does not depend upon the discharge current then the unstable tendency of the glow discharge is reduced, and arc formation is suppressed. This function may be realized by injecting a high energy beam of electrons between the discharge electrodes. Alternatively ionizing electromagnetic radiation from an auxilliary discharge adjacent to the discharge electrodes may be used. A capacitive spark discharge of short duration and of high voltage will provide copious ionizing ultra-violet radiation which will propogate in atmospheric pressure gases for many cm's. Consequently such will provide arc suppression when disposed along the length of the discharge electrode if concomitant with the discharge current.
When the discharge is pulsed many times each second a build up of transient gas species occurs between the dicharge electrodes which are deleterious to the discharge, and which cause arcing. Consequently the lasing gas is blown transversely to the electrodes so as to remove such and prevent build up. Furthermore, commutation of the discharge current at high repetition rate requires special means. The typical switching means for pulsed discharges is the spark gap. The spark previously described as a source of ionizing ultra-violet radiation may be adapted to switch energy storage capacitors to the discharge electrodes. When two metal pin electrodes are immersed in gas of many atmospheres pressure the voltage necessary to cause electrical breakdown between the pin electrodes is many thousands of volts. Moreover when the breakdown or spark occurs a highly conducting channel forms so closing the switch. When a spark gap switch is operated at a high repetition rate ionized gases accumulate between the pin electrodes which causes the breakdown voltage to be reduced. Consequently in the blown (or blast) spark gap which is adapted to high repetition rate the gas is removed from between the pin electrodes with a gas jet. An alternative switching means is the hydrogen thyratron.